Application of partial-response (PR) class-IV (PR4) equalization and maximum likelihood sequence detection (MLSD) has been shown in theory and practice to achieve near optimal performance at recording densities of 0.8&lt;PW50/T&lt;1.6, where PW50 is the pulse width at the 50% amplitude point of the channel's step response and T is the duration of the channel encoded bit. A partial response maximum likelihood (PRML) system for the magnetic recording channel has been described in "A PRML system for Digital Magnetic Recording," Roy D. Cideciyan et al., IEEE Journal on Selected Areas in Communications, Vol. 10, No. 1, pp. 38-56, January 1992. In the U.S. Pat. No. 4,786,890 a class-IV PRML channel using a run-length limited (RLL) code has also been disclosed.
At high recording densities, i.e., PW50/T&gt;1.6, the linear partial response class-IV equalizer leads to substantial noise enhancement. As a consequence, the performance of the PRML detector suffers and may become inadequate to meet product specifications. Application of extended partial-response maximum likelihood (EPRML) detectors has been shown in theory and practice to achieve better performance than PRML detectors in the range PW50/T&gt;1.6. The patent application GB-A-2286952, published on Aug. 30, 1995, discloses a novel EPRML scheme for data detection in a direct access storage device. The novel architecture of the invention claimed therein allows for the addition of EPRML detectors to PRML channels with only minor changes to the overall channel architecture.
The optimum MLSD receiver for detecting an uncoded data sequence in the presence of intersymbol-interference (ISI) and additive Gaussian noise consists of a whitened-matched filter followed by a Viterbi detector which performs maximum likelihood sequence detection on the ISI trellis, as described by G. D. Forney in "Maximum-likelihood sequence estimation of digital sequences in the presence of intersymbol interference," IEEE Trans. Inform. Theory, Vol. IT-18, No. 3, pp. 363-378, May 1972. For the magnetic recording channel the state complexity of this trellis is given by 2.sup.L where L represents the number of relevant ISI terms in the output signal of the whitened-matched filter. In the patent application WO94/29989 with title "Adaptive noise-predictive partial-response equalizing for channels with spectral nulls," filed Jun. 14, 1993 and published Dec. 22, 1994, and in reference "Noise predictive partial-response equalizers and applications," P. R. Chevillat et al., IEEE Conf. Records ICC'92, Jun. 14-18 1992, pp. 0942-0947, it was shown that a partial-response zero forcing equalizer cascaded with a linear predictor whose coefficients have been suitably chosen, is equivalent to the whitening discrete-time prefilter of the optimum MLSD receiver. Furthermore, in the same patent application a receiver structure has been disclosed where the prediction process has been imbedded in the Viterbi detector corresponding to the partial-response trellis. The above patent application WO94/29989 is primarily concerned with wire transmission systems.
In the above patent application WO94/29989 and the article of P. R. Chevillat et al. it has been concluded that noise-prediction in conjunction with PRML improves detector performance.
It is an object of the present invention to provide a method and apparatus with improved data detection performance.